OR, PLAItf TEACHING. 



83 



tion of the head which gives them 

 the most extended range of vision. 

 Fishes breathe by foliated vessels 

 attached to the branchial bones, 

 1, forming gills. The water 

 necessary for respiration enters 

 the mouth, and is driven through 

 the interstices of the branchial 

 bones ; it passes between the 

 membranes forming the gills, the 

 1 vessels of which 



absorb the air 

 contained in the 

 water. Some spe- 

 cies, as well as 

 breathing in this 

 manner, rise oc- 

 casionally to the 

 surface, to im- 

 bibe the free 

 atmosphere; 

 while a few swal- 

 low air, taking 

 it into the intes- 

 tinal canal, from 

 whence it ope- 

 rates upon the 

 blood. The cir- 

 culatory appara- 

 tus of fishes, 

 is simple, the 

 heart havin but 

 one ventricle, 2, 

 and one auricle, 

 3, large veins, 4, 

 8, and arteries, 5, 10, liver, 6, 

 kidneys, 7. 



Although the structure of the heart in fishes 

 i9 so simple, the double circulation is complete. 

 In reptiles, only a part of the black blood 

 passes through the lungs, while in fishes the 

 whole of it traverses the gills. The difference 

 between fishes and air-breathing animals, in 

 respect of circulation, consists merely in this, 

 that the propelling machine has less power : 

 while in mammalia and birds there are two pro- 

 pelling machines (a double heart), one for pro- 

 pelling the blood through the general system, 

 and the other for sending it through the lungs, 

 in fishes a single heart propels it through both, 



343. 



This is because the simple form of fishes re 

 quires a less powerful impulse to be given to 

 the blood. 



Some fishes, instead of having 

 laminated* gills, have either a 

 single open- 

 ing at each 

 side, or a 

 number of 

 small ori- 

 fices,\\. The 

 shark has 

 five pairs of 

 these open- 

 ings, 12, the 

 lamprey, 13, 

 has seven, 

 while the eel, 

 14, has its 

 gills lying 

 within a 



344. 



membranous covering, by which 

 they are kept moist, and shel- 

 tered, so that members of this 

 tribe can live a considerable time 

 out of water. 



Having no other organs for 

 prehension,f they are provided 

 with formidable teeth, 15. The 



braize, 16, has four or six conical 

 teeth in front, with a number of 



* Having layers one over the other, 

 t Taking hold. 



